1. Field of the Invention
The present invention relates to a method and a system for recording and playing back information, and more particularly to a method and a system which utilizes a recording medium having a vertical magnetization film on and from which the information is recorded, and which has a readout mechanism utilizing optical means.
2. Description of the Prior Art
In recent years, recording systems have been proposed wherein information is recorded and played back by utilizing the magneto-optical characteristics of materials such as an amorphous (noncrystalline) alloy used as the recording medium. The amorphous alloy thin films such as Gadlinium Iron (GdFe) and Gadlinium Cobalt (GdCo) are examples of the "vertical magnetization films" in which the direction of magnetization is perpendicular to the surface of the film.
Recording of information on the amorphous thin film takes place in a manner such that the orientation or the direction of magnetization of the desired portions of the amorphous alloy film, which is initially magnetized in a preselected direction, is turned over by heating those portions above the Curie temperature or a compensation temperature while applying a weak external magnetic field.
More specifically, the amorphous alloy thin film which was previously downwardly (corresponding to the binary "0" signal) magnetized is subjected to a weak upwardly biased magnetic field, while only those portions of the thin film on which the "1" signal is to be recorded are applied with a laser beam so as to raise the temperature of those portions above the Curie temperature or a compensation temperature, thereby causing a change in the direction of magnetization.
On the other hand, the read out of information recorded on the thin film takes place by utilizing the magnetic Kerr effect, which is an interaction between the magnetization of substance and a light beam. When a linearly polarized laser beam is reflected by the magnetized thin film, the plane of polarization is rotated in a direction which is determined by the direction of the magnetization. Therefore, the information recorded on the amorphous thin film is read out by detecting the direction of rotation of the plane of polarization of the reflected laser beam by means of an analyzer.
Since this recording medium has the advantage that new information can be rewritten at will, referred to as "reversible recording", so called "editing" of the recorded information becomes possible. In editing, the reading out of the previously recorded information and the erasure of the unnecessary portion of the recording track and rewriting of new information, are usually performed in parallel.
However, the prior art systems of this type have suffered from the problem that certain portions of the recording track are completely erased during editing, which may cause difficulty in tracking the recording beam spot during the recording of new information. That is, there may be a shift in registry between the originally recorded and the newly-recorded information.
In such a case, the discrepancy, or lack of registry, between the newly recorded track and the pre-recorded track results in tracking problems during playback.